1. Field of the Disclosure
The disclosure relates to an optical fiber configured with a noncircular core which is structured to minimize the reduction of the effective area of the fundamental mode along curved stretches of the fiber.
2. Prior Art Discussion
The advent of high power fiber laser systems would not be possible without large mode area (LMA) active fibers. As the power scaling of fiber laser systems grows, strict requirements are applied to the quality of lased radiation. As known to an ordinary skilled worker in the art (“posita”), the fewer the modes guided along the core, the higher the light quality. Hence great efforts are made to configure MM fibers capable of supporting substantially a single, nearly diffraction-limited fundamental mode at the desired wavelength. Once the latter is properly exited, the MM core guides this mode further down a light path without substantial distortion. The latter is largely correct in the context of substantially straight fibers. In practice, however, fibers typically have bends or curved stretches.
Typically for standard fibers, such as fibers with a step-index profile of the core, when a fundamental mode propagates along a curved stretch, its effective area, i.e., a quantitative measure of the area which a mode effectively covers in the transverse dimension, decreases. The displacement of the fundamental mode towards the periphery of the core causes an overlapped area, i.e., the area common to a core region with gain medium and fundamental mode, to shrink. At the same time, the displacement of high order modes towards the core periphery along the bend is not as great as in the case of the fundamental mode. A relatively insignificant displacement of high order modes (HOMs) towards the core periphery along the bend may cause the amplification of the HOM to be substantially greater than the amplification of these modes along the straight stretch of the core. The greater the amplification of HOMs, the lower the quality of the output beam.
As the core area increases, the above disclosed effects become more and more pronounced. These phenomena appear to be one of the main factors limiting the enlargement of the core area and, therefore, the power scaling of active fibers with a step index profile and core diameters at least equal to about 30 μm.
FIGS. 1A and 1B confirm the above. FIG. 1A illustrates the fundamental mode displacement in a 32 μm core extending along a 9 cm radius; FIG. 1B shows the displacement of the mode in a 110 μm core along a 15 cm radius. Clearly, the greater the core diameter, the greater the mode distortion.
The methods of fiber manufacturing and fiber configurations minimizing the bend-induced mode distortion are known. One of the known structures is configured with a parabolic refractive index profile providing for less reduction of the mode area along a fiber bend than that one of the step-index. Also known fiber configurations operating not with a fundamental mode, but with one of higher order modes which each have Δneff—the difference between refractive indices of respective core and mode—higher than that one for the fundamental mode. The higher modes thus are not as susceptible to fiber bends as the fundamental mode is.